I am surprised the rabbits are eating it, it is so bitter, all I know about ragwort is that it is a cumulative poison, so it will not kill them immediately, have you cut it down and dried it a little, that makes it more palatable which may be why they are eating it

Your rabbits are right, ragwort (Marsh at any rate) tastes good, my rabbits just ate some, I tasted it, and it was fairly pleasant, the paca's don't touch it, causes liver failure over a period of time

"Common ragwort (Senecio jacobaea)
has finely divided leaves with a blunt end lobe and appears as a rosette from which a tall flowering spike develops in April/May topped by a large flat topped head with yellow flowers borne on one or more stout stems. It prefers lighter drier, lighter soils, but can occur anywhere.

Marsh ragwort (Senecio aquaticus)
has less divided leaves and a smaller, more irregular head on a shorter, slender stem. It occurs on heavy or poorly drained soil, and is particularly abundant on Orkney; locally elsewhere.

Oxford ragwort (Senecio squalidus)
is similar to marsh ragwort, but the leaves have a pointed end lobe. It is found locally in drier areas, particularly near railway lines and roadsides. It is, however, found more commonly in the south of Britain.

A related weed, Groundsel (Senecio vulgaris)
is also poisonous to stock, but is much more rarely a weed of grassland."

Rhubarb leaves (and the green parts of the stems) contain oxalic acid, so do dock leaves. This is a cumulative poison and it is best to avoid it for rabbits. I don't worry too much if the odd baby dock leaf is in the grass and herbs I cut for mine but never let them eat the bigger leaves or any with rusty coloured spots on them.
Sheep love docks and are ok to eat them, mine make a bee-line for them unless there is ground elder to scoff first, but as a rabbits GI system is so easily disrupted with fatal results I think it's best to be selective about what you feed them. There are so many common herbs, deciduous leaves and cultivars they can eat that it's probably best to play safe and remove the dodgy ones.
Also, if you are keeping them as meat rabbits, wouldn't that mean that you too are ingesting the cumulative toxins?